1. Field of the Invention
The present invention relates to a magnetic mono-component toner for developing an electrostatic latent image (hereinafter simply referred to as “magnetic mono-component toner”) and an image forming method which uses the toner.
More particularly, the present invention relates to a magnetic mono-component toner and an image forming method which uses the toner, wherein with the use of the toner having the coarse powder distribution (S) within a given range in an image forming apparatus which adopts a jumping developing method, a selected developing on a latent image carrier is suppressed and hence, favorable image properties are maintained for a long period.
2. Description of the Related Art
In general, in an electrophotographic method, an electrostatic recording method or the like, a latent image carrier formed of a photoconductive photosensitive body or a dielectric is charged by a corona charge or the like, an electrostatic latent image formed by exposure using laser beams, an LED or the like is visualized using a developer such as toner, or the electrostatic latent image is visualized by inversion developing thus obtaining a high quality image.
Usually, as a toner applicable to these developing methods, toner particles which have an average particle size of 5 to 15 μm and are obtained by a following method is used. That is, a dye or a pigment which constitutes a coloring agent or a charge controlling agent, a wax which constitutes a peel-off agent, a magnetic material and the like are mixed to a thermoplastic resin (a binding resin) which constitutes a binder, and the mixture is blended, pulverized and classified to form the above-mentioned toner particles. Further, to impart fluidity to the toner, to perform a change control of the toner and to enhance the cleaning property, an inorganic fine powdery material or an inorganic metal fine powdery material made of silica, titanium oxide or the like is added to the toner. Further, as a developing method, there have been known a di-component developing method which uses toner and a carrier such as iron powder and a magnetic mono-component developing method which contains a magnetic body in the inside of the toner without using carrier.
To be more specific, there have been known a large number of developing methods which adopt a magnetic brush method described in U.S. Pat. No. 2,874,063 (patent document 1), a cascade developing method and a powder cloud method described in U.S. Pat. specification No. 2,618,552 (patent document 2), and a fur brush developing method. In these methods which use the di-component developer, the methods can provide a relatively stable high-quality image in an initial stage. However, when these methods are used for a long period, the deterioration of the carrier, that is, a spent development arises thus giving rise to drawbacks such as the lowering of the charge imparting ability of the carrier which leads to the difficulty in acquisition of the high-quality image for a long period. Further, these methods have a common defect that it is difficult to maintain a mixing ratio between toner and carrier at a fixed value and hence, the methods lack the durability in a long period.
Accordingly, various developing methods which use a mono-component developer consisting of only toner have been proposed and, particularly, a magnetic mono-component developing method which adopts the magnetic toner is used in general (see U.S. Pat. No. 3,909,258 (patent document 3), JP55-18656A (patent document 4), JP2003-162089A (patent document 5).